Hi. I tightened the 8mm hex bolt to the torque in the instructions (48-54 NM), and now the bearings are really stiff. When I try to spin the cranks, they move but they don't spin freely. Is that normal? Or should I just tighten them until they feel right, like with any other bearing assembly? Thanks.

I figured it out. It's because I was using a spacer on the drive side. My new question, then, is if it is possible to adjust the chainline with this crankset.

I don't see why you should need to...

I have a hub with a 44mm chainline.

A chainline being off by a couple of millimeters will not kill you.

one you shouldn't have to use a spacer unless your frame is weird
two the torque is a measurement of force not depth
three the bearing seals are extremely stiff on these cranks until you put a few miles on them
four try it without the spacer it will probably work fine

That's true, but I'm too neurotic to ignore it.

The omniums are designed so that the left crank arm butts up against the splined part on the spindle, so I don't see any way to use a spacer without putting too much pressure on the bearings.

Ditch the spacers. GXP installation is meant to be basically idiot-proof, don't overthink it.

The spacers aren't for adjusting chainline or overcoming clearance issues. Using them will prevent the non-drive side crank arm from being seted properly on the spindle when tightened to the corrct torque - and crush your bearings in the process.

Best of luck with your neurosis.

^^ haha, Seriously just go the LBS if you cant figure it out.

I did. They told me to just take it an internet forum.

4 times as quickly as they normally do on a GXP bottom bracket, which wear insanely fast as is.

Is that from personal experience? Did you try to use these with a spacer?

That is the one advantage that I hadn't considered about square taper cranks, being able to change the chainline.

It's the reason I won't run a truvativ/sram crank.
You have one size bearing in one cup, a different size bearing in the opposite cup, on a fixed application if you are a bit of an animal on the cranks the gxp setup will last 1/2 as long as a good quality square tapered setup. If you are lighter on the cranks it will last the same amount of time as a comparable setup, sugino 75/DA Track/Record Track.

It's only a 2mm difference in I.D. I think the balls inside the separate cartridge bearings are the same. I've never thrashed a set of cartridge bearings. The benefit of loose ball stuff is that you can repack them with fresh balls periodically, but I have had great luck with simply removing the seals on many "worn" cartridge bearings and thoroughly cleaning and regreasing them to bring them back to life. And if you are patient enough you can repack cartridge bearings by removing the bearing retainer and pushing all the balls to one side to remove the inner race. I guess what I am trying to say is I am a "maximizer."